Automatic sensitivity control circuit



Aug. 18,1936.

REC 7/1 75? R. A. BRADEN v AUTOMATIC SENSITIVITY CONTROL CIRCUIT 3 Sheets-Sheet 1 Filed Oct. 16, 1953 INVENTOR RENE A. RADEN ATTORNEY Aug. 18, 1936.

R. A. BRADEN AUTOMATIC SENSITIVITY CONTROL CIRCUIT Filed Oct. 16, 1933 3 Sheets-Sheet 2 A x If 5min g 7 AF- INVENTOR RENE A. RADEN ATTORNEY Aug. 18, 1936. R, A. BRADEN v 2,051,364

' AUTOMATIC SENSITIVITY CONTROL CIRCUIT Filed Oct; 16, 193 3 3 Sheets-Sheet 3 I XL go l l l l l g Lk INVENTOR RENE A. B ADEN BY MM ATTORNEY Patented Aug. 18, 1936 ICE;

AUTOMATIC SENSITIVITY CONTROI'S' CIRCUIT Rene A. Braden, Collingswood, N; .I'., assignor to- Radio Corporation of America, a; corporation of Delaware Application October 16, 1933, Serial N0.- 693,715

I l-Claims.

lectivity of a radio frequency signalling systemin sucha manner that the system-is more selechire on weak signals than-on strong ones.

Now, in the present application there are disclosed further arrangements for automaticallyvarying. selectivity and improving fidelity when receiving strong signals. In eachof the'present arrangements the tuning of networks resonant to a desired carrier frequency is varied" in: re-

sponseto variations in amplitude of the received signal energy.

Hence, it' maybe stated that it is one of the main objects of the present invention to provide further automatic fidelity control arrangements,

the arrangements functioning to vary the tuning of resonant high frequency networks by'regulating the efiective inductances of certain inductors inthe'resonant network. a

Another objecttof the present invention is to provide an automatic selectivity control circuit for a radio receiver'wherein the rectified signal energy of the receiver is utilized to vary the high frequency response characteristic of a resonant 'circuit preceding the receiver rectifier byadjusting the self-inductance of a control inductor dis:-

posed: in the said resonant circuit. 1

Another object of the present invention is to control the over-all resonance curve of a band -"pass network by adjusting the magnetic coupling between the circuits of. the network, and simultaneously adjusting the band pass circuits.

Still another object of the present invention is 5 toprovide a; receiver with an automatic selectivity control arrangement of the type wherein the output ofa control rectifier is utilized to regulate the tuning. of a pair of cascaded resonant networks in a predetermined manner.

Stillother objects of the invention are to improvegenerally automatic fidelity control? circuits] for radio receivers, and to provide such control circuits which are not only reliable in operation,

but readily constructed and assembled in re- 55""6WrS.

tuning of each of the.

The novel feat'ures which I believe" to be char acteristic of my invention are set forth in particularit'y in-th'e appended claims; the invention itself; however, as to both its" organization and method" of operation, will bestbe understood by 5 reference to the following; description, taken in connection with the drawings, in which I have indicated diagrammatically several organizations whereby my invention maybe carried-into effect.

In: the drawings,

Fig: 1' diagrammaticallyshows-an embodiment of the present invention; I

Figs.- 2 and 3' show two modifications of the arrangementshown in Fig; 1;

Fig; 4 shows another modification of the pres: 15 ent invention, 7

Fig.5 shows a modification of the arrangement in Fig; 4",.

Fig. fishows'a further modification of the circuit in Fig. 4.

Referring now to the accompanying. drawings, wherein like reference characters in the dinerent figuresdesignate similar circuit elements, there is shown in Fig. l in highly conventional form. a radio receiving system embodying one form of the invention. Broadly, it may be stated that the receivingv system shown in Fig. 1 embodies a source of signalenergy, conventionally represented, coupled, as at. M1,.to the resonant input circuit I of a screen" grid amplifier tube I. The anode circuit of? tube l iscoupled, as at M2, to the'resonant input circuit II of a second screen. grid'amplifier tube 2. The output of the amplifier 2 isl'impressed upon a rectifier 3, the input circuit of the rectifier being tuned to the same frequency to which the circuits I and II are tuned; w

The rectifier S-may'bethe customary detector ofthe-receiver, or it may be an auxiliary rectifier tube derivingv its. signal input energy from the network which feeds the conventional receiver detector. Furthermore-,it is to be clearly understood that the stages including, tubes I and 2 maybe operating at intermediate frequency, as in a superheterodyne receiver, or they may be the cascaded amplifi r stages of a tuned radio frequency amplifier receiver. In any case, the. tuning condensers 4 and 5 are to. be understood as resonating their respective circuits'to a desired carrier wave frequency.

Where the rectifier 3' is the detector of the receiver, the reference numeral B designates the anode potential source for'the anode of rectifier 3, the negative side of this source being connected to ground through a resistorR, the circuit be-- tween the anode of the rectifier and the positive side of the source B including a choke and condenser network to block the flow of alternating current energy through the source B. The audio frequency network (not shown) is connected to the-anode. circuit of rectifier 3 through the blocking condenser. To preserve simplicity of description, the energizing potentialsources for the various stages including tubes I and 2 the plate currents of control tubes V1 and V2 flowing through coils L1 and f L2 respectively. These plate currents, in turn, depend on the control biases applied to tubes V1 and V2,"and the control biases are derived from the rectifier anode circuit. 1 7 1 The coil L, as well as the coil L",'is a small iron core coil which is-disposed in its associated.

;; resonant circuit in series between the tuning condenser of the circuit and the coupling transformer. secondary inductor of the circuit. The coils L1 and L2, which carry the plate currents of tubes V1 and V2, are constructed so that they magne- -.jtiz e the cores 6 and I without being coupled to the coils L and L". When the plate currents of tubes V1 and V2 vary, the magnetic flux in each of the cores 6 and-I respectively varies, andcon- 5 R, or fractions of it, is included in the grid'cir- 'current' through L1 increases.

' current'of V1 and V2. that increase insignal strength causes one plate sequently' the self-inductances of coils L and L respectively vary in proportion. As current through L1 increases, flux throughrcore 6 increases, as explained. As flux increases, incremental permeability decreases, due to approach to the condition of magnetic saturation.v The. inductance of coil L is proportional to the incremental permeability, and hence it decreases as This provides. a means of varying the tuning of circuits I and II a small amount.

. The plate currents of tubes V1 and V2 are regulated by connecting the control grid of the tube V1 to the negative side of a grounded bias source 8, and the cathode being connected by a lead 9 to a point on the resistor R. The cathode of" tube V2 is grounded, while its control grid is connected by a leadv III to a point on resistor R. As the signal strength impressed on rectifier 3 varies, the direct current throughR varies, and the voltage drop across R varies. The drop across cuits of V1 and V2, and thus variations in this drop across R cause variations in the grid biases of V1 and V2, and consequent variations in plate Circuits are so arranged current to increase and the other to decrease. Fig. 1, Ip of V1 increases and Ip of V2 decreases,

when signal becomes stronger.

, Taking circuit I as an'ex'ample, when current gflows in L then the inductance of coil L 'is decreased. -With increasing signal strength It of V1 increases, and hence the inductance of L decreases, and the resonant frequency of circuit I becomes higher;

? In circuitII, increasing signal strengthcauses the resonant'frequency to become lower. When so that the I 1" of V1 will be zero, orvery small.

2,051,364 i I i ,7

At the same time, the bias on grid of V2 will have the smallest value that it can have at any time, and I will have its maximum value. Under this condition, circuit I will be resonant at the low frequency end of its range of variation, and

circuit II atthe high frequency end of its range of variation. However, circuits I and II must be adjusted to the same frequency by means of the' condensers 4 and'5, or by altering the other inductances in the amplifier circuits; that is, the

lower end of the range of variation of I must and if. the circuits are constructed properly, each will shift the same number of kilocycles from the initial, common frequency, and the'middrequency of the overall response characteristic will not be altered while the response characteristic itself is made broader in proportion to the strength of the signal.

Summing up, circuit I may be tunedto resonance with the carrier, when receiving weak sig-.

nals, with a small current in tube V1. In this case it is necessary that circuit II he in resonance with the carrier with a high current in tube V2,.

Then, for weak signals tubev V1 has a high grid.

bias, and tube V2 has a low grid bias. As the signal strength increases, the bias on tube V1 de.-' Thus, it

creases and that on tube V2 increases. will be se nthat an increase in signal strength causes the self-inductance of coil L to decrease and that of coil L" to increase. This means that circuits I and II are detuned in opposite directions from the. carrier frequency, thus reducing the selectivity. It will'thus be seen that when the receiving system of Fig. 1 is used to receive strong local stationsignals that the resonance characteristic of the network including tubes I and 2- 'is broadened and fidelity increased, while when tuned, and the coupling M1 between theresonant circuit I and the preceding tuned circuit II is less than critical, the coupling. M2 between the circuit II and its preceding resonant circuit I2 also being less than critical. Otherwise, the circuit elements in the arrangement of. Fig.2 are the same, as in Fig. .1, andtfor this reason the rectifier and its connections to tubes V1 and V2 are not shown.

The arrangement in Fig. 2 operates in the fol-1 lowing manner; the couplings M1 and M1 being less than critical so that, with a weak signal impressed on the system, and circuits H, I2, I and II tuned to the desired signal frequency, the tuned amplifier will be highly selective. 'Then, as the'signalstrength is .made, to increase, circuits I and II are detuned equal amounts in opposite directions from the signal frequency, and thus the over-all response curve is made .broader,

in'proportion to the strength of the signal, while the mid-band frequency, or the frequencyiat the center of the response curve, remains fixed. Of

course, the detuning of the network including tubes I and 2 may be effected on the primary cir-' 75H not the onlyways of accomplishing th'e mode of operation.- represented by' these figures, and: in

Fig. 3 is shown an arrangement 'wlrerein'theneb work, including circuits: l2 and II; between screen grid tubes I and 2 in Fig. 2 is 'controlled:-' In this arrangement of Fig. 3,.asinLFigS. 1'- and 2, thecontrol tubes V1 and V2 have'their plate currents regulated in accordance with'the fiow of plate current inthe anode circuit of rectifier 3, the control on circuitsllZrand II being iniopposite directions. It is believedthat th'e:-diagrammatic showing inFig. 3'will be. sufiicient toillustrate this-modification, since it is substantially the same as in-Fig. 2, with the exception 'that the control tube V1 operatesonwthe circuit? l2 instead of the circuit I,=eliminating -'-the need for operating on two separate stages.

Another means for varying the:selectivity of coupled'tuned circuits is shown inFig. 4; wherein the screen grid tube I is shown asiha vingwits input electrodes connected to a source ofsignals, as in'Fig. 1, while the anode circuit of 'tube is coupled to the input electrodes of the "amplifier tube 2, the latter tube being conventionally shown. The output of tube 2 is tobe understood as being connected to rectifier 3* as-in -Fig'. 1. The coupling network betweentubes l-and 2.'in-

I cludes the tuned circuits 20 and 2|; the-"circuit Y wound on the core 25 a saturating winding 26,

one terminal of the winding being connected to a source of positive potentia-L'not shown; and

the other terminal of the winding being .con-

nected to the anode of control. tube V'2. It is to be understood that the structure comprising 23'," 23'} 25 and- 26 is similar to the structures 6--L1 -L' and l-L2-L" in Fig. 1, with respect to the shape of-the core and the relative-positions of the D. C. coil and the A. C. coils. This means that there is not intended to be anycoupling between coils 23and 26, orbetween 23'" and 26. This provision is made because if such coupling did exist, the plate impedance of V'-2'would be reflected into circuits 20 or 2 I. or both, appearing there as a damping resistance. vThesaturating winding 26 reduces the inductances of coupling coils 23 and 23', and, therefore,-reduces the coupling M and sharpensthe tuning.

Simultaneously, the circuits 20 and2-l are detuned. To correct the tuning, theiron core coils L and L" are provided, these coils-and their control tubes V1 and. V2 being similar to the corresponding elements shown in Fig. 1. When M is reduced by increasing direct current flow through coil 26, the self-inductances of 23 .and

" 23' are reduced also. Reduction in self-inductance of 23 must be balanced by anequal increase of tube V2 is connected to'groundthrough a neg- -ative biasing-source 8"; while its cathode is con- .2 nected to point bon the resistor R of Fig. 1. 1 The scathode of tube V2 is grounded, while its control -grid is "connected to-point a of the resistor shown 'ii1' -'Fig. 1. Ifcircuits 26 and 2| have same L andl C,-eq'ual controlvoltages, as pointed above, should be applied to tubes V1 and V2, and the two cathodes can go to the same point on resistor R (or*Vi can be-connected to coils associated with L and L"; in series, eliminating V2). of V2" '-should go to a separate adjustable tap on R ;-+so that-amount ofinductance change :due

operation of V2 can be made equal (though *bpp'osite) to the inductance change produced by 7 Of course, either a 'the 'o'peration of V1 and V2. p

or '-bconnection may be made at any suitable point along the resistor R.

i With the connections made as described when he saturation' of coil 26 increases, that of coils -L preventing detuning of the coupled tuned circuits ofthe bandpass network. The effect of thevari- 'able coupling is to sharpen or broaden the resonance curve of the coupled circuits, and this is accomplishedby control of the saturating wind- -ing 26'. In efiect there is shown in Fig. 4 a pair o'f tuned circuits coupled by a small iron core transformer with an extra winding 26 carrying 'direct current for saturation. The saturating curvi rent-flowing through the coil 26 is the plate-our- *rent of control tube V2. The griclbias of this control tube is variedin accordance with the rectifiedfidire'ct current potential variations. To cor- 'rect for detuning of the coupled circuits, other saturating iron core elements are inserted inthe coupledcircuits, and these other iron core coils are c'ontrolled by control tubes to preventthe ="d'etuning.

Figure is a modification of the arrangement shownin Fig. 4; wherein an improved arrangement isshown for feeding the control potentials to thetubes Vrand Vz; Since the circuits associated with the-networks-ZO and 2! are substantially the same as those shown in Fig. 4, such networks will not be further described, but it will'be understood that similar reference numerals designate similar circuit elements. A duplex di- "ode-triode tube 55, whose construction is well known to those skilled in the art at the present time, is shown as a source of control potentials for tubes V1 and V2. "the tube 55 are connected to an intermediate point on coil 24', in orderto make a good impedance match. There is arranged a resistor 56 in series between the grounded cathode of tube 55 and the coil L" of the network 2|. current potential component of the rectified voltage developed across resistor 56 is utilized for biasing the control grid of tube V2 by connecting the grid to a point on resistor 56 through a lead 51 which includes the filter resistor 53. lead 51 is adjustably connected to the resistor 56. The audio frequency potential component of' the rectified Voltage developed across resisto-r 56 is impressed upon a succeeding audio frequency am plifier' (not shown) through a lead 59.

The plate of tube V1 is connected to the positive terminal of the voltage source 6El-through the coils El and 62, while the control grid of tube V1 is connected by a lead 63 having an adjustable Grid a'nd L-= decreases, thus keeping the total induct- The two diode anodes of The direct tap to a; desired point .on theresistor R2larrangedw.

' the current through coil 26 decreases and the in theplat'e circuit of tube 55. The entire direct sistor 64.

current potential developed across resistor 551s impressed on the control grid of tube 55 through a path which includes lead- 59 andthe filter re- The operation of the arrangement shown in Fig. 5, with respect to the operation of tubes Vi and V'z is'substantially' the same asin thecase of-Fig. 4-. It is to be noted that the tube V2 has been eliminated. Briefly, the operation of this modification is as follows when signal strength increases, the negative bias applied to the grid of tube V2 is increased.

At the same time, the voltage drop' across resistor 56 is applied to thegrid of the triode section .of tube 55, the audio frequency component being filtered out by the filter resistor 64 and the filter capacity. With increasing signal: strength this bias increases and,'hence, the plate current through resistor R2 decreases. A portion of the voltage drop across R2 is applied as abias to the grid of tube V1. Thus, as signal strength increases,

I the grid of V'z becomes more negative, while that of V1 becomes less negative.

Of course, modifications in the arrangement of Fig. are possible. For example, the triode section of tube 55 may beused to amplify the audio signal component, while the control function performed by. this tube in Fig. 5 may be accomplished with a separate triode. This might be done to avoid connecting the B battery, which is shown in Fig. 5, between resistor R2 and the plate of tube The arrangement shown in Fig. 6 is a'modificationof the arrangement in Fig. 5, insofar as a single tubelfl is employed to perform the functions of tubes V1 and V2 of Fig. 5. The tube 55,

shown in Fig. 5, accomplishes half wave rectification, as in the case of Fig. 5, and the grid of the triode section has both the audio and direct current potential components of the voltage developed across 56 impressed thereon. A resistor ll 7 is arranged in the cathode circuit of tube 55, and :an adjustable tap I2 is connected between ground and anegative point on the resistor H. The coils also decreases.

6| and 62 are connected in series with the screen grid electrode '89 of tube 10, and'positive potential is'supplied' to this screen grid through a ,source 8|. The plate electrode 82 of tube 10 is connected to the positive terminal of resistor II through coil 26 and lead 83. The operation of the arrangement shown in Fig. 6 is as follows:

The plate current of the triode section of tube 55, which is controlled in accordance with signal strength by the bias developedacross resistor 56,

flows through resistor H and produces a voltage drop which is applied to the plate of tube 10. With increasing signal strength the plate voltage of tube 10 decreases and the plate'current Simultaneously, screen current increases with increasing signal strength. Thus,

current through coils BI and 62 increases wit "increasing signal strength.

While I have indicated and described several systems for carryingmy invention into effect, it

will be apparent to one skilled in the art that my invention is'by no means limited to the particular circuits shown and described, but that many modifications may ,be made without departing from the scope of my invention, as s t; forth in the appended claims, 1

What I claim is:-

1. A method of regulating the selectivity of a network including at least two coupled circuits tuned to a desired carrierfrequencmwhich inv 1 eludes the steps of impressing signal energy of said frequency upon the network, rectliying. the impressed energy, and regulating the high frequency response characteristic of. said network with the direct. current component of the rectified energy by adjusting the inductances of said circuits in" a sense to charge the tuning. of the! coupled circuits in opposite directions.

2.- A method. of regulating the selectivity of it network including at least two coupled circuits tuned to. a. desired carrier frequency, which includes the steps of impressing signal modulated-- carrier energy of said frequency upon the: net

work, rectifying the impressed energy, and regulating the high frequency response characteristic of said network with the "direct current component of the rectified energy by varying the effective magnetic coupling between the coupled;

.circuits in a sense inverse to carrier amplitude variation. 7

3. In a receiving system, a pair of amplifier tubes, at least two tuned circuits cascaded between said tubes, said tuned circuits being resonant to a desired signal frequency, a rectifier,

and a'selectivity control means electricallyasso- 'ciated with each of said tuned circuitsQsaid means being responsive to variationsin potential of the direct current componentof the rectified output of said rectifier, said means being conductively connected to the rectifier output circuit and including 'devices operatively associated with the tuned circuits for adjusting the tuningof said tuned circuits in opposite directions with respect to said desired signal'frequency. 7 4. In a receiving system as defined in claim 3, said devices comprising iron core coils disposed in said tuned circuits.

5. In a receiving system as defined in claim 3,

said devices comprising iron core coils disposed in .said tuned circuits, and each selectivity control means comprising a control tube electrically associatedwith each of said coils to vary the magnetic flux of the coil associated with it.;

6. In a receiving system including a'band pass 7 network consisting .of'a pair of magnetically cou-' pled tuned circuits resonant to a common signal frequency, means, responsive to direct current potential variations derived from signal currents,

for varying the intensity of said magnetic, coupling in a sense opposed to signal amplitude variation and in a manner to vary the signal response characteristic of the network.

7. In a receiving system including a band pass network consisting of a pair of magnetically coupled tuned circuits resonant to a common signal frequency, means, responsive'to direct current potential variations derived from signal currents,

for varying the intensity of said magnetic coupling in a sense opposed to signal amplitude variation and in a manner to vary the signal response characteristic of the network, said coupling varying means including a control tube, and

means associated with the coupled tuned circuits for compensating for detuning produced by said pair of resonant cirresonant circuits tuned to a common carrier frequency, an iron core choke connected in each of said circuits, means associated with each of said chokes for regulating the magnetization of the choke cores, and additional means for varying the effectiveness of the last named means.

10. In combination with at least two cascaded resonant circuits tuned to a common carrier frequency, an iron core choke connected in each of said circuits, means associated with each of said chokes for regulating the magnetization of the choke cores, and additional means for varying the efiectiveness of the last named means, said additional means comprising a rectifier network having an input circuit coupled to the second of said resonant circuits, and a control connection between a point in said rectifier network and said regulating means.

11. In combination with at least two cascaded resonant circuits tuned to a common carrier frequency, an iron core choke connected in each of said circuits, means associated with each of said chokes for regulating the magnetization of the choke cores, and additional means for varying the efi'ectiveness of the last named means, said additional means including a diode rectifier network coupled to the second of said resonant circuits, and said regulating means including at least one electrode discharge tube having a cold electrode thereto connected to said diode circuit.

12. In combination with at least two cascaded resonant circuits tuned to a common carrier frequency, an iron core choke connected in each of said circuits, means associated with each of said chokes for regulating the magnetization of the choke cores, and additional means for varying the eifectiveness of the last named means, said regulating means consisting of a pair of electron discharge tubes, each of said tubes being connected to one of said cores, a rectifier network having an input circuit coupled to the second of said resonant circuits, and said rectifier network including an impedance in its space current path, and an elec- 5 trode on each of said tubes and a predetermined point on said impedance.

13. In combination with at least two cascaded resonant circuits tuned to a common carrier frequency, an iron core choke connected in each of said circuits, means associated with each of said chokes for regulating the magnetization of the choke cores, and additional means for varying the effectiveness of the last named means, means for magnetically coupling said resonant circuits, and v a control connection between said additional means and said magnetic coupling means for varying the magnetic coupling between said resonant circuits.

14. In combination with at least two cascaded resonant circuits tuned to a common carrier frequency, an iron core choke connected in each of said circuits, means associated with each of said chokes for regulating the magnetization of the choke cores, and additional means for varying the effectiveness of the last named means, said additional means including an electron discharge tube provided with independent diode and triode sections, means for coupling the diode section to the second of said resonant circuits to provide a diode rectifier circuit, a connection between a grid of said triode section and a point in said diode circuit, an impedance in the space current path of said triode section, and a control con- 5 nection between said impedance and said regulating means.

RENE A. BRADEN.

Certificate of Correction Patent No. 2,051,364. August 18, 1936. RENE A. BRADEN It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows: Page 2, first column, line 66, for L, read L page 4, second column, line 7, claim 1, for charge read change; and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Oflice.

Signed and sealed this 23rd day of March, A. D. 1937.

[SEAL] HENRY VAN ARSDALE, Acting Commissioner of Patents. 

